As a next-generation system of the W-CDMA (Wideband Code Division Multiple Access) and the HSDPA (High Speed Downlink Packet Access), an LTE (Long Term Evolution) system has been studied by 3GPP (3rd Generation Partnership Project) which is a standards body of the W-CDMA. In the LTE system as a radio access system, an OFDMA (Orthogonal Frequency Division Multiplexing Access) scheme and an SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme have been studied to be applied to downlink communications and uplink communications, respectively (see, 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA”, June 2006, for example).
The OFDMA scheme is a multi-carrier scheme in which a frequency band is divided into plural sub-carriers having narrower frequency bands and data are transmitted on the respective sub-carriers. The sub-carriers are closely arranged in the frequency direction, allowing the sub-carriers to be partly overlapped without causing interference, so that fast data transmission can be achieved and frequency usage efficiency can be improved.
The SC-FDMA scheme is a single-carrier scheme in which a frequency band is divided so that different frequencies can be separately used among plural terminals (user equipment terminals) and as a result, interference between terminals can be reduced. Further, in the SC-FDMA scheme, a range of transmission power fluctuation can be made smaller; therefore lower energy consumption of terminals can be achieved and a wider coverage area can be obtained.
Typically, the capabilities of mobile stations are defined in the mobile communication system. For example, as shown in FIG. 1, twelve types of capabilities (User Equipment capabilities or User Equipment categories) are defined in HSDPA (see 3GPP 25.306 V6.8.0 2006-03, Table 5.1a, for example). In HSDPA, a maximum number of receivable codes, a minimum TTI reception interval, a maximum transport block size, and a soft buffer size for an HS-DSCH (High Speed Downlink Shared Channel) are defined as the capabilities.
As used herein, the soft buffer size refers to an intermediate soft buffer size for 2-Stage Rate Matching in HARQ (Hybrid Automatic Repeat reQuest). The relationship between the 2-Stage Rate Matching and the soft buffer size is described below in detail. In HARQ, when signals are combined according to Incremental Redundancy, a buffer needs to be reserved for three signals (a Systematic bit, a Parity 1, and a Parity 2) for turbo coding. In this case, the size of the buffer (the memory size) to be reserved is three times larger than the size of actually received data, which presents a problem of increasing the workload in the mobile station. In order to solve the problem, 2-Stage Rate Matching is used. Specifically, the mobile station performs Rate Matching to a predetermined buffer size (First Rate Matching), and then performs further Rate Matching (Second Rate Matching) to generate bits to be mapped to the physical channel. The combination of signals according to Incremental Redundancy is made based on a bit sequence after First Rate Matching (see 3GPP 25.212 V6.9.0 2006-09, 4.5.4, for example). Since the mobile station may reserve the buffer size corresponding to the bit sequence after First Rate Matching, the buffer size (memory size) can be reduced.
The reason for defining the capabilities is described below. For example, HSDPA is a communication system which achieves a maximum transmission rate of 14 Mbps. When the capabilities are not defined, in other words, when only one capability is used, all the mobile stations must have the capability of the maximum transmission rate of 14 Mbps. On the other hand, the mobile communication system or the mobile communication service using the mobile communication system provides various mobile stations ranging from an expensive and high-performance mobile station to a low-cost and low-performance mobile station, in order to satisfy various user needs. Without the capabilities, the service may be degraded. Accordingly, the capabilities are defined in order to satisfy various user needs for the various mobile stations ranging from the expensive and high-performance mobile station to the low-cost and low-performance mobile station.
As shown in FIG. 2, for example, the capability is reported from the mobile station to the base station apparatus at the start of communications in HSDPA. Specifically, the mobile station transmits a connection request (RRC Connection Request) to the base station apparatus (S1) and the base station apparatus transmits a connection setup (RRC Connection Setup) to the mobile station (S3). Then, the capability (User Equipment radio access capability), which is one of information elements in a connection complete (RRC Connection Setup Complete) used for confirming establishment of the connection, is reported from the mobile station to the base station apparatus (S5). In this manner, the capability is specified from the mobile station to the base station apparatus (see 3GPP 25.331 V6.11.0 2006-09, 10.2.41, for example).